Image sensors such as those used in digital cameras and other optical or image sensing equipment are conventionally housed in a ceramic package. The ceramic package includes a ceramic frame which is epoxy bonded to a glass lid or cover. The ceramic package is expensive and not readily adapted to manufacture in strip form or other multiple unit form, as is widely employed in the semiconductor packaging industry. In addition, the use of epoxy as a bonding agent presents several problems such as moisture penetration through the epoxy bond, outgassing of the epoxy which can contaminate the semiconductor device, and air leakage which limits the ability to hermetically seal the ceramic package. Further, it is difficult to accurately align the glass cover to the ceramic frame so that the glass cover is parallel to the image sensor surface. This alignment difficulty is caused by an inability to control the thickness of an epoxy bead which is commonly employed to seal the glass lid to the ceramic frame.
A conventional ceramic sensor package is shown in sectional elevation view in FIG. 1. This package includes a two layer ceramic substrate composed of an outer ceramic substrate 10 and an inner ceramic substrate 12, on the upper surface of which is disposed a CCD or CMOS image sensor 14. A ceramic frame 16 defines the cavity 18 in which the image sensor is located. The ceramic frame is hermetically sealed to the peripheral surface of the substrate 10 and a boro-silicate glass lid or window 20 is sealed to the upper surface of frame 16 by a UV curable adhesive. The contacts of the image sensor are wire bonded to contacts 22 provided on the inner surface of substrate 10. These contacts 22 are electrically connected to outer contact pads 24 by conductive feedthroughs or vias provided in substrate 10. The feedthroughs 26 are typically plated with gold to enhance the electrical conduction between the inner and outer contact areas. A UV curable adhesive is usually employed to bond the glass window to the ceramic package frame to prevent exposure of the semiconductor sensors to high temperatures which are needed to cure other types of epoxy adhesives but which can degrade or destroy the semiconductor sensors. The requirement for UV curable adhesive materials limits the range of available epoxies which can be employed in the conventional ceramic package, since most epoxy adhesives are curable at elevated temperatures.
The reliability of semiconductor and other electronic device or component packages including image sensor packages is related to the “airtightness” or hermeticity of the package. Hermeticity is a measure of an ability of the package to protect the semiconductor or other device housed in the package from an entrance of fluids and moisture. Moisture or corrosive gases on or near a semiconductor device can cause corrosion of the metallic traces on the semiconductor device, and can lead to failure. Traditional hermetic packages are made from metal, ceramic, or vitreous materials. These materials have such low permeabilities that moisture and fluids typically are impeded by these materials, and cause a condensation on the semiconductor device or contamination by corrosive gasses.
In addition to a permeation of fluids or moisture, fluids or moisture can penetrate the image sensor package though “leaks” at several interfaces. The interfaces for a ceramic package include a metal/ceramic interface, along with epoxy/glass and epoxy/ceramic interfaces. Any small openings at these interfaces allow fluids or moisture to seep inside the image sensor package.